Images with -1 Downvotes

https://derpibooru.org/search?q=downvotes%3A-1
 
Over 600 images on Derpi have -1 downvotes somehow. I checked the forums and found a thread about the issue which also said the issue was fixed… three years ago.
 
I’m not saying I want this to be removed, the presence of negative downvotes is kind of endearing if anything, but how did it happen in the first place? Deleting downvotes from people who went on downvote sprees?

@Background Pony #A511  
Likely some of the jobs for mass vote removal from spanners herped due to the amount. But the root cause should be fixed.